N. Ireland does more trade with the rest of the UK than with the Republic, and so is concerned that customs disruption would be relatively greater. I have not seen any figure for EU trade via the Republic, however, so it is possible the above statistic is misleading.

Psychologically, moving the customs border to Irish sea could be seen as a precursor to moving the Sovereign border there - something which is obviously totally unacceptable to them, and indeed their whole raison d'être. The DUP are hypersensitive to anything that might be construed as putting distance between Britain and N. Ireland, as "internal" customs posts in the Irish sea would be seen as doing.

The future of Northern Ireland after Brexit can only be secured if it remains in the customs union, a former secretary of state will tell the House of Lords on Tuesday.

The Labour peer Peter Hain, who as Northern Ireland secretary helped steer the peace process when the IRA was decommissioning its arms, will tell the Lords that the government's position on Northern Ireland is "delusional, contradictory and potentially very damaging".

Lord Hain told the Guardian that the government's negotiating strategy in Brussels was incompetent. He was particularly scathing about its recently published position paper on Northern Ireland and Ireland, which suggests that an "invisible" border could be maintained if the EU agrees to waive checks on small businesses providing goods and services, including dairy and meat products.

He said the government knew that this was "pie in the sky" because EU law would require Ireland to have checks to prevent products such as hormone-injected beef or chlorinated chicken from the US from making its way into the EU.